For the protection of repeaters, relay and other types of equipment interposed in transmission lines and the like, and thus having both input and output terminals, it is a recognized practice to employ two gas surge arresters, one connected to the input, and the other to the output terminals with both also connected to the ground terminal. This configuration provides ionizable gaps between the input line(s) and ground, and the output line(s) and ground. The object of these arrangements is to provide protection of the repeater, etc. from surges, whether they originate upstream or downstream of the equipment.
While the surge arresters at both the input and output terminals may have equivalent ratings so far as breakdown potential is concerned, in practice the actual voltage necessary to ionize the tubes will vary. For example, a tube with a strike voltage rated at 350 volts may actually strike at voltages as low as 300 volts or as high as (and sometimes higher than) 500 volts.
As a consequence of this range of uncertainty respecting the striking voltage, conditons can occur wherein, in the presence of a surge originating at either the input or output terminals of the equipment, the difference in striking voltages between the input and output arresters is sufficiently different such that an excessive current may flow through the equipment causing damage. This danger has become increasingly acute with the proliferation of very low power solid state circuits.
While this problem may be mitigated by employing matched tubes, this is a relatively expensive proposition both in original installation cost and in overhead, maintenance and replacement costs. Further, some deterioration in matching can occur with time.
It is accordingly an object of the invention to provide a protector arrangement for equipment having input and output terminals, which does not depend upon matching, and which is relatively inexpensive, easy to install and easy to replace, and which minimizes the development of damaging differential surge currents.
Other objects and advantages of the invention will be apparent from the following description or will be learned in the course of practicing the invention.